1. Field of the Invention
The invention relates to a sensor arrangement, comprising a sensor and an a signal processing circuit for processing the output signal of the sensor, having at least one first current source, which is connected to an operating voltage and by means of which a current is passed through the sensor.
2. Prior Art
European Patent Disclosure EP 0 049 304 A1, for instance, discloses using inductive sensors for simultaneous measurement of a travel or motion and the temperature. A sensor with at least one coil is used, and the coil changes its electrical properties as a function of the measurement variable, such as a displacement and the prevailing temperature.
In the known arrangement, an inductive spacing measuring device with a measuring coil is employed. The measuring coil is acted upon on the one hand by a high- frequency alternating current and on the other by a direct current or a low-frequency current. From the induced eddy current losses, the impedance of the coil and thus in the final analysis the position of the measuring coil can be determined.
Because of the ohmic resistance of the measuring coil, a voltage drop occurs at the coil that is further processed, with the aid of a low-pass filter, to a direct voltage value. Since the ohmic resistance of the measuring coil varies with the temperature, the voltage value is at the same time a measure of the temperature of the measuring coil and can be utilized for compensation of the actual measurement signal.
From European Patent Disclosure EP 0 332 196 B1, it is known to use an inductive sensor to measure the rotational speed, or rpm, of a wheel in an ABS-equipped vehicle brake system. The associated evaluation circuit takes into account the fact that the electromagnetic signals induced in accordance with the rpm increase considerably in both amplitude and frequency with the rpm and therefore provides suitable compensatory provisions. It is also taken into account that the internal resistance of the rpm sensor varies as a function of temperature.
To increase the accuracy of the evaluation, a temperature compensation is provided; it is proposed that either the temperature of the rpm sensor be ascertained with a separate sensor, or the temperature dependency of the internal resistance of the rpm sensor be determined in advance and stored in memory as a calibration curve in the evaluating microprocessor. There is no proposal that the rpm and temperature be measured separately. Nor is it known to use the temperature of the rpm sensor as a measure for the temperature of the brakes.
A sensor arrangement described at the outset is also known from German Patent Disclosure DE 44 31 045 A1. The known sensor arrangement is used to jointly measure two variables, in particular the rpm of a wheel and a temperature, with an inductive pickup that is disposed in the vicinity of the wheel and is influenced by the wheel. The associated evaluation circuit ascertains the rpm from the chronological spacing between pulses of the output signal of the inductive pickup. The temperature is ascertained from the voltage offset that occurs. The sensor arrangement is used as a wheel rpm sensor in a motor vehicle and is disposed in the vicinity of the brakes; a temperature dependent on the brake temperature is measured and evaluated.
Although the known sensor arrangement also provides very good results, it still has the disadvantage that its advantageous effect is typically achieved only with inductive sensors, in which the frequency of an induced or generated alternating voltage represents the measurement signal. It can therefore advantageously be used where the measurement signal by an oscillating circuit mistuning or an rpm- dependent flux, as is the case in DE 44 31 045 A1. If the measurement signal is generated by a cross coil force sensor, then the known sensor arrangement cannot be used.
The object of the invention is to provide a sensor arrangement of the type defined at the outset in such a way that it can be used to evaluate measurement signals in which the amplitude of an alternating voltage represents the measurement signal.
This object and others, which will be made more apparent hereinafter, are attained in a sensor arrangement comprising a sensor that produces an alternating voltage sensor signal dependent on a measured variable, which appears across the sensor terminals, and an evaluation circuit for evaluating the sensor signal, which has at least one current source, which is connect to an operating voltage and by means of which a current is passed through the sensor.
According to the invention the sensor circuit arrangement comprises a sensor for generating the alternating voltage sensor signal, which has an internal sensor resistance and two sensor terminals; a first current supplying circuit connected to an operating voltage and to one sensor terminal to provide a first output voltage at that sensor terminal; a second current supplying circuit connected to the operating voltage and to another sensor terminal to provide a second output voltage at that sensor terminal; a counter-coupling network connecting a control input of the first current supplying circuit and a control input of the second current supplying circuit and also connecting the sensor terminals with each other, the counter-coupling network comprising means for coupling the first and second current supplying circuit, so that the output voltages are set to respective predetermined values; and a sensor signal processing circuit portion comprising an operational amplifier having input terminals connected across the sensor terminals and an output terminal, a low-pass filter connected to the output terminal of the operational amplifier and having a filter output signal indicative of the internal sensor resistance and a capacitor connected to the output terminal of the operational amplifier and generating an alternating output signal having an amplitude representing a measured variable.
By means of the second current source, it is advantageously possible to dispose the sensor between the two current sources. Since the current of the current sources can be set by means of the countercoupling network, the output voltages of the current sources can assume predetermined values. As a result, the potential at which the sensor outputs its signal can be set. The sensor is thus no longer at a fixed potential but rather at an ungrounded, floating potential. Because of the freedom in the potential, the major advantage is attained that common-mode interference reaching a differential amplifier that amplifies the sensor signal can be compensated for.
The countercoupling network also prevents the current sources from reaching saturation, which would cause their outputs to be at either nearly the potential of the operating voltage or at ground potential. By means of the internal regulation of the countercoupling network, the circuit arrangement according to the invention has the capability not only of suppressing common-mode interference but also of improved constancy of the current. This is very favorable from the standpoint of more-accurate temperature measurement. Since the sensor is not attracted to potential on one side, short-circuit and breakaway detectors can also remain active.
Particularly in a force sensor in the form of a cross duct, whose secondary coil can be evaluated in the manner described above, the sensor arrangement of the invention has major advantages, since a cross duct has the property, given fixed wiring on one side, of having low sensitivity. If the primary side of the cross duct were also applied on one side to a fixed potential, there would be the additional risk of miscoupling. These disadvantages are avoided in a sensor arrangement embodied according to the invention.
It is especially favorable if the countercoupling network is designed such that the value of the output voltages of the current sources is approximately half the operating voltage. As a result, a very wide control range is obtained.
It has proved especially favorable that the countercoupling network carries the output voltages of the current sources each to a respective summation point in the control input of the current sources. This makes it especially easy to realize the circuitry for the internal regulation. It is also very favorable if the countercoupling network has a low-pass filter, whose limit frequency is below, or markedly below, the frequency of the alternating signal furnished by the sensor. The effect attained by the low-pass filter is that the countercoupling network allows only the low-frequency signal components for controlling the current sources to pass through to the control input of the current sources, and it blocks the higher-frequency sensor signal.